You make a lot of it's/its mistakes. Remember, its is the one you use as a pronoun, it's is the one you use for a possession. Well, that's how I remember it anyways. You might use a different mnemonic.

I feel like you slip in and out of scientific tone, but I could also be tired. Just in case, take a look at Clinical Tone Declassified for a good guide to scientific tone. It's a bit of a read, but well worth it. Also, your sentence structure can be a bit wonky, but again, tired.

"5m x 5m x 10m" is not a cube. Cubes are equal on all sides.

I don't know which idea of Keter you're using here. I usually go with the locked-box test. What is preventing us from putting all the anomalous spiders in a box, locking the box, and calling it a day? Maybe I'm a little tired, but I didn't see what was preventing us from doing that.

This is really, really long. I'm not sure if that's a good thing or a bad thing. I'll probably edit this with my opinion when I'm more awake.

Your first hurdle for me concept-wise is that SCP-2031 did the hell out of the hive-that-mimics-its-victims, and we also have a couple different takes on communal spiders, and SCP-632 which are brain spiders in the purest sense. You've still got an open niche, though by focusing on the unique parts here: First, they don't mimic or puppeteer their victims, or just eat the brain, but instead utilize the brain until they get hungry. Second, they have that whole the thing, but with spiderwebs thing going on, which has some good gee-whiz factor and some mild science-horror.

Execution isn't bad persay, but it's bogged down with a lot of over-description and lack of focus. I have the same problem a lot of times. The solution is to drag out the hedge trimmers. Let's start with the containment proceedures.

Italics=extraneous info

Special Containment Procedures: SCP-() is to be contained in a 5m x 5m x 10m concrete chamber , with observational windows.

Exact measurements probably aren't important, nor is what the room is made of. They don't breath acid or fire, and probably aren't going to chew through the walls if they're made of steel plates instead of concrete for some reason. If the chamber is sealed and well reinforced (which you can pretty much assume as standard) they aren't going anywhere. There's no need for observation windows. They might be useful if such a chamber is available, but they aren't a vital part of containment.

and a 2m x 1m hatch in the ceiling for inserting sheep into the chamber for feeding. The hatch is to have a secondary hatch behind it and both are to be electrified. Under no circumstances should both hatches be open at the same time.

Diet and feeding schedule are useful, not so much how you get it in the room. A ceiling hatch probably isn't the best option, either, even if you electrify it. Spiders can climb and spider silk isn't a great conductor. Just put the sheep through the airlock. The last sentence is a pretty obvious safety thing, and wouldn't be needed either way. Also, a sheep seems a bit excessive for at most 2 kg of spiders.

The chamber is to be studded with nozzles of no larger than 3mm at intervals of 50cm for the purposes of administering gasses to the chamber for the purposes of ventilation and suppression.

If the world won't end if you use 6mm nozzles at 100 cm or a closed-system rapid air exchanger instead, you can just say "a system for the release of whatever gas".

The floor is to be a scale with a resolution of 0.01g to allow the measurement of SCP-() colonies.

This could be a useful feature of chambers in general, but unless these things will escape if you don't have an exact reading on their mass at all times, it's not vital to containment.

Any entry to the chamber must be performed in a full body hazmat suit after SCP-() instances have been suppressed with desflurane gas.

Gas bit should probably be with the part about the gas system, but otherwise this is more what you want.

The chamber must be cleaned bi-annually of waste products and deceased instances of SCP-().

And this is toward the "good" type of fluff; simple stuff relevant to the unique containment needs of the SCP. I'd still lean toward leaving it out though.

In the event that the total mass of SCP-() colonies exceeds 2kg, desflurane gas is to be administered to the chamber through air nozzles to suppress SCP-() colonies, and D-Class personnel in hazmat suits are to remove and incinerate SCP-() instances until the mass of SCP-() colonies falls below 500g. In the event this procedure can not be performed, protocol 'Arachne' shall be put into effect.

You can trim and reword this to reduce it quite a bit.

Any test involving the exposure of live humans to SCP-() requires authorisation of a member of the O5 council. No D-Class personnel exposed to SCP-() should have an IQ exceeding 130 and should have no knowledge related to the containment of SCP-(). If this is not the case, protocol 'Arachne' shall be put into effect.

IQ isn't a very useful statistic, D-class testing is more of a matter for the ethics committee, and all of this only really matters if they actually get the information.

So your containment procedures only really need to be something like:

SCP-() is to be contained in a sealed standard containment chamber equipped with a desflurane gas release system for sedation purposes. Entry to the chamber must be performed in a full body hazmat suit after SCP-() instances have been sedated.

Once per {However often you have to feed them} a single {I'd suggest a rat instead of a sheep}is to be introduced to the chamber for feeding purposes.

A minimum of 500g breeding stock of SCP-()is to be maintained at all times. The total mass of SCP-() colonies is not to exceed 2kg; excess SCP-() instances are to be culled and incinerated.

In the event that culling can not be performed, or should SCP-() obtain access to an individual or information deemed to pose a risk to containment, protocol 'Arachne' shall be put into effect.